Centralizers are known for positioning tubulars, such as casing, drill pipes, rod strings and the like, within wellbores to minimize wear between the tubular and the wellbore walls in the case of an openhole application, or between the tubular and the casing walls in a cased wellbore, regardless the orientation of the wellbore. Without a centralizer, wear may be enhanced in horizontal wellbores particularly at the heel or deviated portion of the wellbore where directional changes would otherwise cause the tubular to engage the casing. Further, without a centralizer, grinding of the casing may occur when portions of the tubular, such as the drill pipe tool joints, are hard-faced with stiff material such as tungsten carbide. Efforts are made to make the hard-facing as smooth as possible so as to minimize the casing wear but since the drill pipe rotates it is inevitable that drill pipe tool joints will wear against the casing.
Generally, the centralizer engages the tubular and acts to space the tubular from either the wellbore walls or the casing. Casing centralizers are generally one piece and slide over the casing. Tubulars, such as drill pipe, have tool ends formed thereon and therefore drill pipe centralizers must be clamped over the tubular and secured thereon.
One such centralizer available from Hawkeye Industries Inc. of Edmonton, Alberta, Canada comprises a discontinuous, molded urethane, tubular body which is sufficiently flexible to be installed about a tubing string. The tubular body has molded fins extending radially outwardly therefrom to space the tubing string from the casing or wellbore walls. The centralizer is secured about the tubing string using a stainless steel band clamp. When the fins on the centralizer body have worn such that they no longer provide sufficient offset to space the tubular from the wellbore or casing walls, the centralizers are discarded and replaced.
Another tubing centralizer is available from Western Well Tool Ltd., of Houston Tex., USA. A tubular body of the centralizer comprises a plurality of hinged segments which are pinned together to encircle the tubular. Opposing end collars abut uphole and downhole ends of the tubular body for positioning the centralizer along a length of the tubular. The end or thrust collars, generally comprise two arcuate segments which are bolted together about the tubular to form the thrust collars. The thrust collars sandwich the tubular body therebetween in the axial direction. The bolts are typically high tensile steel bolts. Applicant understands that the hinged segments and the pins which connect the segments to form the body are prone to failure with repeated use. Failure of centralizers can be costly, particularly if portions of the centralizer fall into the wellbore and disrupt operations therein.
Yet another example of a prior art tubing centralizer is the RotoTEC® centralizer available from Tercel Oilfield Products, Dubai, UAE. The RotoTEC® centralizer comprises a freely rotating outer sleeve positioned over an internal pipe sleeve. The sleeves are made of a composite material such as a self-lubricating polymer with a low coefficient of friction and appear to comprise at least two “clamshell” halves which are pinned together in order to be positioned over the tubing. Upper and lower retaining clamps are bolted about the tubing string for retaining the sleeves thereon in the axial direction. When the outer sleeve is worn such that it no longer provides sufficient standoff to space the tubular from the wellbore or casing walls, at least the outer sleeve must be discarded and replaced.
In another aspect, there are competing objectives for centralizers in openholes and cased wellbore environments. While the centralizers must provide adequate standoff, centralizers running against casing are expected to minimize wear of the casing string. The nature of such centralizers is that the resilient nature thereof renders the softer portion of the centralizer vulnerable to wear and failure in openhole scenarios.
There is interest in the industry for a simple, efficient tubing centralizer and methodology for drilling that considers the environment for cased wellbores and open portion of horizontal wells.